Sensibility of tourmaline chemistry to granitic magma composition and oxygen fugacity

Abstract

Extensive granitic magmatism is a dominant feature of the Central Iberian Zone (CIZ) of the Variscan Orogen. For the most part, these are S-type peraluminous granitic rocks exhibiting variable degrees of evolution and compositions ranging from granodiorites, through mon zogranites and granites to leucogranites, bearing either biotite and muscovite, or just muscovite in the more evolved facies. Tourmaline is a common and essential accessory mineral in many of these peraluminous granitic rocks. Several granite-hosted tourmaline sets from the Castelo Branco, Idanha-a-Nova and Penamacor Monsanto plutons were used to investigate how tourma line chemistry reflects granitic magma composition and oxygen fugacity. Additionally, previously published data on tourmalines and their host-granites from Rebordelo (CIZ, Portugal) and the Alamo Complex and several Araya-type granitic batholiths (CIZ, Spain) were used to test the trends obtained. Most tourmaline components and component ratios, however, seem substantially impervi ous to granitic magma composition and oxygen fugacity. Exceptions are the Mg/(Mg + Fe) ratio and Ti contents of tourmaline, which show evident variation with the degree of evolution and oxygen fugacity of host granitic rocks. From both mineralogical and petrological point of view, it seems of interest that these compositional features of tourmaline may be used as indicators of the degree of evolution and of specific characteristics of the granitic magmas that produced them